385 research outputs found
Endovenous treatment of the greater saphenous vein using a 1,320 nm Nd:YAG laser causes fewer side effects than using a 940 nm diode laser
The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. II. NLTE spectral analysis of the OB-type subdwarf Feige 110
In the framework of the Virtual Observatory (VO), the German Astrophysical
Virtual Observatory (GAVO) developed the registered service TheoSSA
(Theoretical Stellar Spectra Access). It provides easy access to stellar
spectral energy distributions (SEDs) and is intended to ingest SEDs calculated
by any model-atmosphere code, generally for all effective temperature, surface
gravities, and elemental compositions. We will establish a database of SEDs of
flux standards that are easily accessible via TheoSSA's web interface.
The OB-type subdwarf Feige 110 is a standard star for flux calibration.
State-of-the-art non-local thermodynamic equilibrium (NLTE) stellar-atmosphere
models that consider opacities of species up to trans-iron elements will be
used to provide a reliable synthetic spectrum to compare with observations.
In case of Feige 110, we demonstrate that the model reproduces not only its
overall continuum shape from the far-ultraviolet (FUV) to the optical
wavelength range but also the numerous metal lines exhibited in its FUV
spectrum.
We present a state-of-the-art spectral analysis of Feige 110. We determined
, and
the abundances of He, N, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, and Ge. Ti, V,
Mn, Co, Zn, and Ge were identified for the first time in this star. Upper
abundance limits were derived for C, O, Si, Ca, and Sc.
The TheoSSA database of theoretical SEDs of stellar flux standards guarantees
that the flux calibration of astronomical data and cross-calibration between
different instruments can be based on models and SEDs calculated with
state-of-the-art model-atmosphere codes.Comment: 19 pages, 7 figure
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV observations of some massive globular clusters uncovered a significant
population of very hot stars below the hot end of the horizontal branch (HB),
the so-called blue hook stars. This feature might be explained either as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich
stars show strong carbon enrichment as predicted by the late hot flasher
scenario. We conclude that the helium-rich HB stars in omega Cen cannot be
explained solely by the helium-enrichment scenario invoked to explain the blue
main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
Chemical Abundances and Rotation Velocities of Blue Horizontal-Branch Stars in Six Globular Clusters
High-resolution spectroscopic measurements of blue horizontal-branch stars in
six metal-poor globular clusters -- M3, M13, M15, M68, M92, and NGC 288 --
reveal remarkable variations in photospheric composition and rotation velocity
as a function of a star's position along the horizontal branch. For the cooler
stars (Teff < 11200 K), the derived abundances are in good agreement with the
canonical cluster metallicities, and we find a wide range of v sin i rotation
velocities, some as high as 40 km/s. In the hotter stars, however, most metal
species are strongly enhanced, by as much as 3 dex, relative to the expected
cluster metallicity, while helium is depleted by 2 dex or more. In addition,
the hot stars all rotate slowly, with v sin i < 8 km/s. The anomalous
abundances appear to be due to atomic diffusion mechanisms -- gravitational
settling of helium, and radiative levitation of metals -- in the non-convective
atmospheres of these hot stars. We discuss the influence of these photospheric
metal enhancements on the stars' photometric properties, and explore possible
explanations for the observed distribution of rotation velocities.Comment: 77 pages, 27 figures, accepted for November 2003 publication in ApJ
Clusters AgeS Experiment. Hot subdwarfs and luminous white dwarf candidates in the field of the globular cluster M4
We present UBV color magnitude diagrams (CMDs) for the globular cluster M4.
The CMDs show a sequence of four luminous blue stars (V<20, U-V<-0.6) which
appear to be cluster hot subdwarfs. We present spectra for the three brightest
ones. We also note the presence of a population of faint blue objects, likely
to be hot, young white dwarfs (WDs) belonging to the cluster. We have selected
five objects above V=22 mag, bright enough for follow-up ground-based
spectroscopy and present their coordinates and finding charts. We show a
spectrum for variable V46 (Kaluzny et al. 1997) which suggests that it is a hot
subdwarf, along with a new light curve obtained with the ISIS image subtraction
package (Alard 2000). The light curve is unstable, but only one period of
variability is apparent. Two new variables have been discovered, both located
on the cluster red giant branch (RGB). We also present a differential E(B-V)
reddening map and a fiducial sequence for the main sequence, subgiant branch
and red giant branch on the V/B-V CMD for a selected region with uniform
reddening. Based on a comparison with the M5 fiducial sequence we obtain a
reddening estimate of E(B-V)=0.41 mag towards M4, consistent with previous
determinations.Comment: 16 pages LaTeX, 14 PostScript figures, 5 tables, major revisions,
accepted to A
Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction
While the near-infrared wavelength regime is becoming more and more important
for astrophysics there is a marked lack of spectrophotometric standard star
data that would allow the flux calibration of such data. Furthermore, flux
calibrating medium- to high-resolution \'echelle spectroscopy data is
challenging even in the optical wavelength range, because the available flux
standard data are often too coarsely sampled. We will provide standard star
reference data that allow users to derive response curves from 300nm to 2500nm
for spectroscopic data of medium to high resolution, including those taken with
\'echelle spectrographs. In addition we describe a method to correct for
moderate telluric absorption without the need of observing telluric standard
stars. As reference data for the flux standard stars we use theoretical spectra
derived from stellar model atmospheres. We verify that they provide an
appropriate description of the observed standard star spectra by checking for
residuals in line cores and line overlap regions in the ratios of observed
(X-shooter) spectra to model spectra. The finally selected model spectra are
then corrected for remaining mismatches and photometrically calibrated using
independent observations. The correction of telluric absorption is performed
with the help of telluric model spectra.We provide new, finely sampled
reference spectra without telluric absorption for six southern flux standard
stars that allow the users to flux calibrate their data from 300 nm to 2500 nm,
and a method to correct for telluric absorption using atmospheric models.Comment: Reference spectra available at CDS. Published in A&A 568, A9, 201
Hot UV-bright stars of galactic globular clusters
Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. Aims. We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory. Methods. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of -2.3†< †[Fe/H]†< †-1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Results. We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low. Conclusions. Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.Fil: Moehler, Sabine. European Southern Observatory; AlemaniaFil: Landsman, W. B.. National Aeronautics and Space Administration; Estados UnidosFil: Lanz, T.. Observatoire de la Cote D'Azur; FranciaFil: Miller Bertolami, Marcelo Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentin
Developmental regulation of voltage-gated K+ channel and GABAA receptor expression in Bergmann glial cells
Bergmann glial cells are closely associated with neurons: during development they provide guiding structures for migrating granule cells and in the adult cerebellum they display intimate interactions with Purkinje cells. In this study, we have addressed the question of whether such changes in neuronal-glial interactions during development are accompanied by variations in the membrane properties of Bergmann glial cells. We used a mouse cerebellum slice preparation to study membrane currents of the Bergmann glial cells at various stages of development in situ using the patch-clamp technique. The distinct morphology of Bergmann glial cells was revealed by Lucifer yellow injections during recording. While Bergmann glial cells in mice of postnatal day 20 (P20) to P30 have thick processes with arborized, irregularly shaped leaf-like appendages, the processes of cells from younger mice (P5-P7) are thinner and smoother. This morphological maturation is accompanied by a variation in voltage-gated currents. In cells from P5 to P7, delayed outward- and inward-rectifying K+ currents were recorded, while older Bergmann glial cells were characterized by, large, voltage- and time-independent K+ currents. In addition, application of GABA induces two effects, a rapid activation of a Cl- conductance and a longer-lasting decrease in the (resting) K+ conductance. Both effects were mediated by benzodiazepine-insensitive GABAA receptors. Responses in cells of P5-P7 mice were large as compared to the small or even undetectable responses in P20-P30 cells. These GABAA receptors were characterized immunohistochemically in mice and rat brain sections with five subunit-specific antibodies. Bergmann glial cells exhibit a distinct but transient immunoreactivity for the GABAA receptor alpha 2-, alpha 3-, and delta-subunits. Staining is maximal between P7 and P10 and decreases gradually thereafter. In contrast, antibodies to the alpha 1- and beta 2,3-subunits fail to decorate Bergmann glial cells, although they yield a prominent staining of both the Purkinje cells and the granule cells. These changes in the Bergmann glial cell membrane properties and GABAA receptor expression suggest a transition between functional states during development of the Bergmann glial cells
Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)
We present a spectral analysis of the sdO central star K 648 based on
medium-resolution optical and high-resolution UV spectra. The photospheric
parameters are determined by means of state-of-the-art NLTE model atmosphere
techniques.
We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08)
and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a
factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a
sub-solar value. However, these metal abundances are much higher than the
cluster's metallicity M 15: [Fe/H] = -2.25).
The surface composition appears to be a mixture of the original hydrogen-rich
material and products of helium burning (3 alpha process) which have been mixed
up to the surface. The abundances of He, C, and N are consistent with the
nebular abundance, while O is considerably more abundant in the photosphere
than in the nebula.
From a comparison of its position in the log Teff - log g plane with
evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity
of 3810 +/- 1200 Lsun are deduced.
Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the
distance of M 15 as determined by Alves et al. (2000).
From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure
Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
We use high-resolution spectra obtained with the ESO Very Large Telescope to
measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc.
We discovered that a sub-population of BSS shows a significant depletion of
Carbon and Oxygen with respect to the dominant population. This evidence would
suggest the presence of CNO burning products on the BSS surface coming from a
deeply peeled parent star, as expected in the case of mass-transfer process.
This is the first detection of a chemical signature clearly pointing to a
specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5
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